High-efficiency scintillation detector for combined of thermal and fast neutrons and gamma radiation

ABSTRACT

A scintillation based radiation detector for the combined detection of thermal neutrons, high-energy neutrons and gamma rays in a single detecting unit. The detector consists of a pair of scintillators sandwiched together and optically coupled to the light sensitive face of a photomultiplier tube. A light tight radiation pervious housing is disposed about the scintillators and a portion of the photomultiplier tube to hold the arrangement in assembly and provides a radiation window adjacent the outer scintillator through which the radiation to be detected enters the detector. The outer scintillator is formed of a material in which scintillations are produced by thermal-neutrons and the inner scintillator is formed of a material in which scintillations are produced by high-energy neutrons and gamma rays. The light pulses produced by events detected in both scintillators are coupled to the photomultiplier tube which produces a current pulse in response to each detected event. These current pulses may be processed in a conventional manner to produce a count rate output indicative of the total detected radiation even count rate. Pulse discrimination techniques may be used to distinguish the different radiations and their energy distribution.

This invention, which is a result of a contract with the U.S. Departmentof Energy, relates generally to the art of radiation detection fordetecting neutrons and gamma radiation and more specifically toscintillation based radiation detection.

BACKGROUND OF THE INVENTION

There are several applications such as space research, health physics,and subcriticality experiments where there is a need for ahigh-efficiency radiation detector that is sensitive to neutrons over awide energy range and gamma radiation. The need for such a detectorarises from various requirements including physical limitations, costeffectiveness, and simplicity.

SUMMARY OF THE INVENTION

In view of the above need, it is an ohject of this invention to providea high efficiency single radiation detector which can he used to detectboth thermal and fast neutrons and gamma radiation.

Other objects and many of the attendant advantages of the presentinvention will be obvious to those skilled in the art from the followingdetailed description taken in conjunction with the drawing.

In summary, the invention pertains to a scintillation based radiationdetector for detecting both thermal and fast neutrons and gammaradiation including a first scintillator responsive to thermal neutrons,a second scintillator responsive to fast neutrons and gamma radiationarranged in a sandwiched assembly optically coupled to the lightsensitive face of a photomultiplier tube. A portion of the tube and thescintillators are enclosed within a light tight, radiation pervioushousing. Radiation entering through a front window formed by the housingfirst interacts with the first scintillator which is formed of amaterial having a thickness sufficient to stop all thermal neutronswhile allowing fast neutrons and gamma radiation to pass therethroughinto the second scintillator which has a thickness sufficient to stop alarge fraction of the fast neutron and gamma radiation. Scintillationsproduced in both scintillators are detected by the photomultiplier tubewhich produces current pulses at a rate corresponding to the totaldetected radiation event rate. These pulses may be processed in aconventional manner to provide a readout of the total combined radiationevents detected. Pulse shaping discrimination can be used to distinguishthe fast neutrons, thermal neutrons, and gamma rays. In addition, theenergy spectra of the fast neutrons and gamma rays may be measured.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a schematic diagram illustrating a scintillationdetector made in accordance with the present invention for detectingboth thermal and fast neutrons and gamma radiation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, a detector 5 made in accordance with thepresent invention is shown. The detector includes a thin glassscintillator 7 responsive to thermal neutrons and optically coupled to athick plastic scintillator 9 responsive to high-energy neutrons andgamma rays. The plastic scintillator is optically coupled to the lightsensitive face 11 of a photomultiplier tube 13 by means of a lightcoupling lens 15. The lens 15 is formed with a flat front face which isdisposed against the back side of scintillator 9 and a concave backsurface which is disposed against the light-sensitive, convex facesurface 11 of the photomultiplier 13. The scintillators 7 and 9 areassembled in a sandwiched arrangement with the lens 15 using an opticalcoupling compound, such as a silicone based compound No. C-20057available from the Dow Corning Corporation, Midland, Mich., between eachelement and the face 11 of the photomultiplier 13. This arrangement isheld in assembly by means of a rectangular, light-tight housing 17 whichsurrounds the scintillators 7 and 9 and a portion of the outer, opaquemagnetic shield 19 of the photomultiplier tuhe assembly. The assembleddetector arrangement is inserted into the aluminum housing with theouter scintillator 7 against the front portion of the housing forming aradiation pervious detector window 21 through which the radiation to bedetected enters the detector. The housing is held in place against thephotomultiplier assembly by means of a hack cover plate 23 having acentral opening which fits about the rearward extending neck portion 25of the housing 17 in a press fit arrangement against a foam rubberspacing and insulting collar 27. The collar 27 is shaped to compressagainst the shoulder portion 29 of the photomultiplier shield 19 andform a light tight fit between the housing and the tube shield 19 whenthe cover 23 is attached to the housing wall portion, as shown. Anadditional foam rubber spacing collar 31 may be inserted between theshield 19 and the tube body 13 at the shoulder region to preventdisplacement of the shield 19 relative to the tube body 13.

The output of the photomultiplier tube, availahle at a rear connector 33of the tube, consists of current pulses produced at a rate correspondingto the total combined radiation event count rate and an amplitudeproportional to the energy of the detected radiation event. This outputmay be connected to a pulse processing system to count or store thepulse information as in a conventional detection system. For example,the output may be connected through a broadband amplifier 35 to oneinput of a fast discriminator 37. The amplitude of the pulses from theamplifier is compared to a reference voltage applied to the referencevoltage input of the discriminator 37 and produces an output count pulsefor each input pulse which exceeds the reference voltage threshold. Thediscriminator reference is typically set at a voltage which eliminatesthe counting of noise pulses produced in the photomultiplier 13. Thecount pulses from the discriminator may be registered in various ways asby feeding the pulses directly to a display pulse counter 39 or storingthe pulses in digital form in a computer based analyzer for lateranalysis.

Referring again to the scintillators 7 and 9, the particularscintillating materials used may vary as long as the requirements ofdetection for each medium is met. Specifically, the first scintillator 7must be formed of a material having a large thermal neutroncross-section and a thickness sufficient to absorb all thermal neutronsof the radiation to be detected while allowing high-energy neutrons andgamma rays to pass therethrough. Similarly the second scintillator 9must be formed of a material having a large cross-section forhigh-energy neutrons and gamma rays and sufficient thickness to absorbthese radiations. Thus, it will be seen that a scintillation is producedin the first scintillator 7 for each detected thermal neutron and ascintillation is produced in the second scintillator 9 for eachhigh-energy neutron or gamma ray detected. The light pulses produced hyscintillations in each scintillator are optically coupled to thephotomultiplier to produce output current pulses indicative of the totalcombined radiation detection rate.

In the illustrated emhodiment, the first scintillator 7 is formed of anoptical quality glass doped with lithium at a concentration of at least7.5% which has been enriched in lithium-6 (⁶ Li) to at least 95%. Thisscintillator is 5.9 inches square and 5 mm thick. A scintillator of thistype is manufactured hy the Levy West Co., a British company, andavailable through the BICRON Corp., Newherry, Ohio. It is identified asa glass scintillator, Type KG2 and is available in the size specifiedabove. The second scintillator is formed of an appropriate organicplastic scintillating material having a large cross-section forhigh-energy neutrons and gamma rays. One such scintillator is the ModelNo. BC-420 plastic scintillator available from BICRON Corp. This plasticscintillator has a 6-inch square cross-section and is 4 inches thick,which is sufficient for the detection of high-energy neutrons and gammarays. Other plastic or liquid scintillators such as NE 213 may also heused for the second scintillator which is responsive to both high-energyneutrons and gamma ray radiations.

In the arrangement shown the thicker plastic scintillator 9 also servesas a light guide for guiding the light pulses produced by scintillationsoccurring in the front glass scintillator 7 to the photomultiplier face11. The photomultiplier used is a standard 5-inch nominal diameter tubesize.

Tests of the above described detector were made using calibrated sourcesof neutrons and gamma rays to determine the detector efficiency, orefficiency factor, i.e. the number of counts produced per unit number ofradiations that impinge upon the detector. Thermal neutrons havingenergies up to 0.2 eV are detected with an efficiency of ≧95%, those upto 1 eV with an efficiency of ≧75%, and those up to 4 eV with anefficiency of ≧50%. Fast neutrons having energies of fission neutronspectrums in the range of from 0.2 to 5 MeV were detected with detectorefficiency factors of at least 11%. Gamma radiation from a source havingan energy of 0.66 MeV was detected with a detector efficiency factor ofat least 15%.

Thus, it will be seen that a highly efficient detector has been providedfor the combined detection of both thermal and fast neutrons and gammaradiations in a single detector. In the past, individual detectors havebeen required to detect each type of radiation. The dual scintillatordetector may be used for any application where high efficiency isrequired. It has considerable advantage over conventional detectors fordetecting all energies of neutrons and gamma rays from fission reactionsor accelerator targets. The detector is extremely valuable wherephysical limitations near or inside experimental facilities prevent thepositioning of multiple detectors.

Although the invention has been illustrated by means of a specificembodiment, it will be obvious to those skilled in the art that variousmodifications and changes may be made therein without departing from thespirit and scope of the invention as set forth in the claims appendedhereto.

We claim:
 1. A radiation detector for the combined detection of thermalneutrons, high-energy neutrons and gamma ray radiations, comprising:afirst scintillator means disposed to receive said radiations enteringsaid detector through an entrance area thereof for producingscintillations in response to thermal neutrons absorbed therein andallowing said high energy neutrons and gamma rays to pass therethrough;a second scintillator means disposed adjacent said first scintillatormeans to receive said high-energy neutrons and gamma rays passingthrough said first scintillator means for producing scintillationstherein in response to high-energy neutrons and gamma rays absorbedtherein and optically coupled to said first scintillator means forguiding light pulses therethrough generated by said scintillationsproduced in said first scintillator means; a photomultiplier tube havinga light sensitive face optically coupled to said second scintillatormeans for receiving light pulses generated by said scintillationsproduced in said first and second scintillator means and producingcurrent pulses at an output thereof in response to each of saidgenerated light pulses indicative of the total combined radiation countrate; and a light-tight housing disposed about said first and secondscintillator means and said light sensitive face of said photomultiplierand having a radiation pervious window therein forming said entrancearea of said detector.
 2. The radiation detector as set forth in claim 1wherein said first scintillator means includes a ⁶ Li doped glassscintillator having sufficient thickness to absorb said thermal neutronradiation.
 3. The radiation detector as set forth in claim 2 whereinsaid second scintillator means includes an organic plastic scintillatorhaving sufficient thickness to absorb both said high-energy neutrons andsaid gamma ray radiations.
 4. The radiation detector as set forth inclaim 3 further including an optical coupling lens disposed between saidsecond scintillator means and said light sensitive face of saidphotomultiplier tube and wherein said first and second scintillatormeans and said coupling lens are serially aligned in a sandwichedassembly between said radiation entrance window of said housing and saidlight sensitive face of said photomultiplier tube.
 5. The detector asset forth in claim 4 wherein said housing is disposed about a portion ofsaid photomultiplier tube in a light tight arrangement to hold saidfirst and second scintillator means and said optical coupling lens inassembly against said light sensitive face of said photomultiplier tube.6. The detector as set forth in claim 5 further including recordingmeans responsive to said current pulses at the output of saidphotomultiplier tube for recording said pulses as an indication of thetotal detected radiation count rate.